Multilayer reflective films having non-linear spacing of layers

ABSTRACT

A reflector for reflecting a broad bandwidth of electromagnetic (EM) radiation, comprising a sheet comprising a large plurality of pairs of layers of transparent polymer material parallel to a surface of the sheet, each pair of layers having a difference in the index of refraction between the materials in each layer of the pair, the total thickness of each pair of layers in the large plurality of layers varying substantially continuously and non linearly across the thickness of the sheet, is disclosed.

RELATED CASES

[0001] This is a Continuation-in-part of copending application Ser. No.08/805,603 entitled “Electro-optical glazing structures havingtotal-reflection and transparent modes of operation for use in dynamicalcontrol of electromagnetic radiation” by Sadeg M. Faris and Le Li, filedFeb. 26, 1997, which is a continuation-in-part of: copending applicationSer. No. 08/739,467 entitled “Super Broadband Reflective CircularlyPolarizing Material And Method Of Fabricating And Using Same In DiverseApplications”, by Sadeg M. Faris and Le Li filed Oct. 29, 1996, which isa Continuation-in-Part of copending application Ser. No. 08/550,022 (NowU.S. Pat. No. 5,691,789) entitled “Single Layer Reflective SuperBroadband Circular Polarizer and Method of Fabrication Therefor” bySadeg M. Faris and Le Li filed Oct. 30, 1995; copending application Ser.No. 08/787,282 entitled “Cholesteric Liquid Crystal Inks” by Sadeg M.Faris filed Jan. 24, 1997, which is a Continuation of application Ser.No. 08/265,949 filed Jun. 2, 1994, which is a Divisional of applicationSer. No. 07/798,881 entitled “Cholesteric Liquid Crystal Inks” by SadegM. Faris filed Nov. 27, 1991, now U.S. Pat. No. 5,364,557; copendingapplication Ser. No. 08/715,314 entitled “High-Brightness Color LiquidCrystal Display Panel Employing Systemic Light Recycling And Methods AndApparatus For Manufacturing The Same” by Sadeg Faris filed Sep. 16,1996; and copending application Ser. No. 08/743,293 entitled “LiquidCrystal Film Structures With Phase-Retardation Surface Regions FormedTherein And Methods Of Fabricating The Same” by Sadeg Faris filed Nov.4, 1996; each said Application being commonly owned by Reveo, Inc, andincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates generally to reflective film havinga large plurality of extruded or coextruded layers, where the thicknessof the layers varies non-linearly throughout the thickness of the filmto produce a very broad band reflective film. Such films may be used togreat advantage in an electro-optical glazing structure havingreflection, semi-transparent, and transparent modes of operation whichare electrically-switchable for use in dynamically controllingelectromagnetic radiation flow in diverse applications.

[0004] 2. Brief Description of the Prior Art

[0005] The use of multilayer polymer films for controlling reflectivityhas been known for many years. Such films comprise many layers,generally alternating between two types of transparent polymer, eachhaving different refractive indices and an appropriate thickness relatedto the wavelength of the light to be controlled. U.S. Pat. No.3,711,176, by Alfrey, Jr. et al. details theoretical details of such afilm. U.S. Pat. No. 3,610,729, by Howard Rogers introduces a multilayerpolarizer, where each alternate layer is birefringent, where the indexof refraction for light of a first linear polarization differs fromlayer to layer and that linear polarization is reflected, and the indexof refraction for light of the second linear polarization is the samefrom layer to layer and the second linear polarization light istransmitted. The bandwidth of the light reflected from such multilayerfilms is generally limited to a small portion of the bandwidth ofvisible light (20 nanometers in the case of the Alfey patent. Also, ifinfra red reflecting film is required which is transparent in thevisible region, higher order effects occur to produce unwanted reflectedcolors from the film. U.S. Pat. No. 5,103,337, by Schrenk et al.proposes using more than two different materials to control unwantedhigher order effects. U.S. Pat. No. 5,686,979, by Weber et al., proposesto use multilayer reflecting polarizing film as a “smart window” for thecontrol of light by reflecting the light. The reflectivity, however, isgenerally limited to a narrow bandwidth and such films are not equallytransparent outside of the reflective bandwidth of the films. Generalreferences on polymer dispersed liquid crystals may be found in detailin “Polymer Dispersed Liquid crystal displays”, by J. W. Doane, achapter in “Liquid Crystals”, Ed. B. Bahadur, World ScientificPublishing, Singapore, and “CLC/polymer dispersion for haze-free lightshutters, by D. Yang et al. Appl. Phys. Lett. 60, 3102 (1992). SmartWindow Design is treated in “Electrochromism and smart window design”,by C. Granqvist, Solid State Ionics 53-56 (1992) and “large scaleelectochromic devices for smart windows and absorbers”, by T. Meisel andR. Baraun, SPIE 1728, 200 (1992). The above identified US patents andreferences are hereby incorporated by reference.

OBJECTS OF THE PRESENT INVENTION

[0006] It is an object of the invention to provide a reflectivemultilayer polymer film having a very wide bandwidth.

[0007] It is an object of the invention to provide a reflectivemultilayer polymer film having little variation in the reflectivityoutside of the reflective bandwidth of the film.

[0008] It is an object of the invention to provide a polarizingreflective multilayer polymer film having a very wide bandwidth.

[0009] It is an object of the invention to provide a polarizingreflective multilayer polymer film having little variation in thereflectivity outside of the reflective bandwidth of the film.

[0010] It is an object of the invention to provide a “smart window”using a polarizing reflective multilayer polymer film having a very widebandwidth.

[0011] It is an object of the invention to provide a “smart window”using a polarizing reflective film having a very wide bandwidth combinedwith a reflective multilayer polymer film having a very wide bandwidth.

[0012] It is an object of the invention to provide a “smart window”using a polarizing reflective film having a very wide bandwidth combinedwith a reflective multilayer polymer film having little variation in thereflectivity outside of the reflective bandwidth of the film.

[0013] It is an object of the invention to provide a “smart window”using a polarizing reflective multilayer polymer film having a very widebandwidth combined with a light scattering layer for further control oftransmitted light.

SUMMARY OF THE PRESENT INVENTION

[0014] The present invention provides a reflective film comprising alarge plurality of pairs layers of transparent polymer, each layer ofthe pair having a different index of refraction. The light reflectedfrom the polymer interfaces add coherently to give high reflectivity.The layer thicknesses change through the thickness of the film in asubstantially continuous and non-linear way, so that a very broad bandwidth of light may be reflected and so that higher order effects areminimized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a sketch of a prior art multilayer polymer reflector.

[0016]FIG. 2 shows the distribution of thickness of layers of prior artmultilayered polymer reflectors as a function of depth into themultilayer film.

[0017]FIG. 3 shows the non linear distribution of thickness of layers asa function of depth into the film of the method of the presentinvention.

[0018]FIG. 4 shows a glazing panel for a “smart window” using theapparatus of the invention

[0019]FIG. 5 shows a glazing panel for a smart window using theapparatus of the invention combined with an additional panel for furthercontrolling light transmitted through the “smart window”.

[0020]FIG. 6 shows a glazing panel for a smart window using theapparatus of the invention combined with an additional scattering panelfor further controlling light transmitted through the “smart window”.

[0021]FIG. 6 shows a glazing panel for a “smart window” combined with anadditional panel using the apparatus of the invention for furthercontrolling light incident on the “smart window”.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0022]FIG. 1a shows a sketch of the prior art multilayer reflector film10. Pairs of layers of two different transparent polymer materials 12and 14 are arranged adjacent with each layer adjacent the next layer.Such films are generally co-extruded and pulled so that the thickness ofthe layers is in the submicrometer range. For a reflector, the layersshown in FIG. 1a are equal to one quarter of an optical wavelength (thewavelength of the light to be reflected divided by the index ofrefraction of the material for light of that wavelength). An incidentlight ray 16 is shown impinging on the film 10, and many reflections 18are shown reflecting from the interface of the two transparent polymermaterials 12 and 14. The “wavelength” of designed reflectivity isusually designed for light incident normally on to the film 10. FIG. 1ashows the light slightly off normal incidence, and for large angles ofincidence the wavelength of maximum reflectivity shifts to longerwavelengths because of the longer path length of the light in thematerials 12 and 14. The light rays 16 and 18 are shown unrefracted atthe air material interface for clarity of presentation.

[0023]FIG. 1b shows a sketch of the thickness of the material layers asa function of depth into the film for the stack shown in FIG. 1a.

[0024]FIG. 2a is a sketch of a prior art multilayer reflector whichattempts to make the reflector broad band. The prior art literatureproposes a monotonic increase in the thickness of the layers. The layerthickness increases by step function in FIG. 2a. Each stack of differentthicknesses reflects a different narrow band of light. The reflectioncoefficient is not constant over the reflective bandwidth of the film. Aproblem arises when the stack shown in FIG. 2a is used for an infraredreflecting film to cover a window. Second order effects produce avisible color in the film. This problem is addressed in the cited U.S.Pat. No. 5,103,337, by Schrenk et al., who proposed films having threedifferent materials to do away with second order reflection effects.FIG. 2b shows a sketch of the layer thickness as a function of depthinto the film.

[0025] The apparatus of the present invention is shown in FIG. 3a. Thetransparent polymer material layers have thicknesses which increase intothe depth of the film in a non-linear way. It is shown in great detailin copending application Ser. No. 08/739,467 entitled “Super BroadbandReflective Circularly Polarizing Material And Method Of Fabricating AndUsing Same In Diverse Applications”, by Sadeg M. Faris and Le Li filedOct. 29, 1996, which is a Continuation-in-Part of copending applicationSer. No. 08/550,022 (Now U.S. Pat. No. 5,691,789) entitled “Single LayerReflective Super Broadband Circular Polarizer and Method of FabricationTherefor” by Sadeg M. Faris and Le Li filed Oct. 30, 1995, that a nonlinear distribution of the twist of a cholesteric liquid crystal (CLC)polarizer is vital to producing a broad band reflector having areflectivity which varies little over the reflection band. Such CLCreflectors also have smooth reflectivity profiles in the second order ifthe CLC material is used to reflect infrared light.

[0026] To use the CLC broadband reflectors of the prior applications toreflect all the incident light in a particular bandwidth, of two typesof CLC must be used. One type reflects right hand polarized (RHP) light.The other reflects left hand polarized (LHP) light.

[0027] In the present invention, the reflectivity at the interfaces ofthe two types of materials is not necessarily dependent on thepolarization of the incident light. However, the two types of materialshown in FIG. 3a may include birefringent materials as detailed in U.S.Pat. No. 3,610,729, by Howard Rogers. In this case, the multilayerpolarizer resulting would be very broad band. Such polarizers could beused to great effect in the “smart window” applications of copendingapplication Ser. No. 08/805,603 entitled “Electro-optical glazingstructures having total-reflection and transparent modes of operationfor use in dynamical control of electromagnetic radiation”by Sadeg M.Faris and Le Li, and U.S. Pat. No. 5,686,979, by Weber et al.

[0028]FIG. 4a shows the film of the invention 10 used to reflect a firstbandwidth of light, for example the infrared bandwidth, while the “smartwindow” of the above identified references is used to control thetransmission and reflection of a second bandwidth of light (for examplethe visible portion). Smart windows are typically a glazing structurecomprising two polarization dependent transparent sheets 40 a and 40 bsandwiching a transparent conduction material 42 such as indium tinoxide (ITO) and a polarization control material which changes stateunder an electric field 44. Support materials such as glass panes arenot shown for clarity. Light of the first bandwidth 46 is reflected atthe film of the invention 10, while light in the second bandwidth passesfilm 10. In one embodiment of the smart window using multilayerpolarizers to reflect light, light of a first linear polarization isreflected from layer 40 a, while light of the second polarization istransmitted through the polarization control material 44. Depending onthe voltage imposed across the polarization control material, thepolarization state of the light transmitted is controlled so that thelight is reflected or the light is transmitted (FIG. 4c) by polarizationdependent layer 40 b. Other embodiments of smart windows shown in detailin copending applications may controllably transmit or reflect all ofthe incident light in a broad bandwidth.

[0029]FIG. 4c shows a sketch of an additional embodiment of theinvention, whereby an additional controllable layer is added to thesmart window structure of FIG. 4a. It is very difficult to ensure that100% of the light across the entire visible wavelength is reflected, andthe privacy of occupants of a room with such a smart window may becompromised. An additional structure 49 is used to control the lightwhich is transmitted through the smart window. The embodiment shown inFIG. 4c uses a scattering layer as layer 49. Layer 49 may also be anabsorptive layer, for example and electrochromic panel or absorptivepolarizers. Electrochromism is define as a reversible optical absorptionchange induce in a material by an electrochemical redox process. Theelectrochromic device employs two electrochromic materials (ECM) with“complimentary” properties. The first electrochromic material isnormally reduced and undergoes a colorless-to-colored transition uponreduction. The second electrochromic material is oxidized and undergoesa similar color transition upon gain of electrons. Most electrochromicsmart windows adopt a thin film configuration. The two complementaryelectrochromic materials are coated on two opposite electrodes andremain there during the redox coloration process.

[0030] The film of the invention may be used as a polarization reflectorin a smart window using linear polarization. This embodiment may be usedwith or without the film 10 shown in FIG. 1a. The embodiment shown inFIG. 4 c may also be used with or without the film 10. This embodimentmay also be used with a passive infrared reflecting film comprising twoCLC layers, each reflecting opposite polarizations. This embodiment mayalso be used with an infrared reflecting film comprising flakes embeddedin a transparent medium.

[0031] Controllable scattering structures are shown in detail incopending applications which are included by reference. FIGS. 5a-c showa novel structure, whereby flakes 52 of the film of the invention aresuspended in a material 54. The orientation of the flakes may becontrolled by an electrical field. The light incident upon thescattering structure is shown to be reflected, transmitted, or scatteredin FIGS. 5a, 5 b, and 5 c respectively, depending on the orientation ofthe flakes. The flakes are typically of dimensions of ten to fiftymicrons in diameter. Use of such light controlling flakes for CLCmaterials is shown in copending application Ser. No. 08/787,282 entitled“Cholesteric Liquid Crystal Inks” by Sadeg M. Faris. Flakes of the filmof the invention may be used suspended in any transparent material asreflecting paint. The broad bandwidth of the film is useful inreflecting light which is incident at a large angle of incidence to thesurface of the film.

[0032]FIGS. 6a and 6 b show an embodiment of a scattering layer, whichshows light rays 60 incident on a layer 62 which contained an polymermaterial 63 contained between two transparent electrically conductinglayers 42. Contained within the polymer are regions 66 of liquid crystalmaterial formed into small spheres of micron or submicron dimension.Such a material is called a polymer dispersed liquid crystal (PDLC). Themolecules of the liquid crystal material, sketched in FIG. 6a as shortlines, are correlated by the internal forces in the liquid crystal tohave internal order, which may be random from droplet to droplet asshown in FIG. 6a. Light propagating through the polymer material 63strikes the droplet of liquid crystal material 66, and will in generalrefract at the polymer liquid crystal interface because there willgenerally be a change in the index of refraction of the (randomlyordered) liquid crystal material and the polymer material. The layer 62will then scatter light passing through.

[0033] The light rays traced in FIG. 6a are shown scattered andtransmitted through the layer 62, which would be the case for very lightloading of liquid crystal material in the polymer. In the more generalcase, light incident on the panel would be as likely scattered backwardas forward, and would likely be scattered isotropically in alldirections.

[0034]FIG. 6b shows the results of applying an electric field across thelayer 62 by applying voltage across the conducting layers 42. Theelectric field forces the liquid crystal molecules in each sphere toline up parallel with the field. In this case, the index of refractionof the liquid crystal material matches the index of the polymermaterial, and the light rays pass through the layer 62 without deviationor scattering.

[0035] An alternative embodiment for a scattering layer is sketched inFIGS. 7a-c. The liquid crystal material is admixed with a polymermaterial, but unlike the case of FIG. 6a, the resultant material doesnot phase segregate. The linear liquid crystal molecules remainentangled in the polymer material, which can be thought of as a mass ofwet, springy spaghetti. FIG. 7a shows the liquid crystal molecules asshort lines. In the case of FIG. 7a, the molecules are lined up parallelwith the conducting plates because, for example, the surfaces of theplates have been rubbed. In the example shown, the internal order of themolecules also aligns then to reflect incident light on the layer 73.When an electric field is impressed across the layer as in FIG. 7b, themolecules rotate to line up parallel to the field, and light propagatingparallel to the field passes through the 1 a without scattering,reflection, or absorption. When the electric field is turned off, thepolymer acts as a restoring force to rotate the molecules back to theirstarting position as shown in FIG. 7a.

[0036]FIG. 7c shows the mixture when no external order is imposed on theliquid crystal material by a rubbed alignment layer or by electricfield. The liquid crystal material still wants to lower its internalenergy by having near neighbor molecules align with one another, butthere is no long range order. The regions of material now scatter lightrandomly and, with no field applied, the light incident on the layer 74is scattered. When an electric field is impressed, the molecules swingaround to line up with the field, and the light passes through withoutscattering as in FIG. 7b.

We claim:
 1. A reflector for reflecting a broad bandwidth ofelectromagnetic (EM) radiation, comprising: a sheet comprising a largeplurality of pairs of layers of transparent polymer material parallel toa surface of the sheet, each pair of layers having a difference in theindex of refraction between the materials in each layer of the pair, thetotal thickness of each pair of layers in the large plurality of layersvarying substantially continuously and non linearly across the thicknessof the sheet.
 2. The reflector of claim 1, where the broad bandwidth ofthe electromagnetic spectrum is in the infrared portion of theelectromagnetic spectrum.
 3. The reflector of claim 2, furthercomprising: an electro-optical glazing structure having reflection andtransmission modes of operation for selectively reflecting andtransmitting electromagnetic radiation, respectively, theelectro-optical glazing structure comprising: an electro-optical glazingpanel having first and second optical states of operation; optical stateswitching means for switching the electro-optical panel to the firstoptical state of operation in order to induce the electro-opticalglazing structure into the reflection mode of operation, and forswitching the electro-optical panel to the second optical state ofoperation in order to induce the electro-optical glazing structure intothe transmission mode of operation.
 4. The electro-optical glazingstructure of claim 3, further comprising a controllable scatteringlayer.
 5. The electro-optical glazing structure of claim 3, wherein thecontrollable scattering layer comprises a fluid medium containing alarge plurality of anisotropically shaped objects for controllablyscattering light, the orientation of anisotropically shaped objectscontrollable by a field.
 6. The electro-optical glazing structure ofclaim 3, wherein the controllable scattering layer comprises a polymermedium containing a large plurality of inclusions, each inclusioncontaining liquid crystal material, the liquid crystal materialcontrollable by a field.
 7. The electro-optical glazing structure ofclaim 3, wherein the controllable scattering layer comprises a mixtureof a polymer medium and a liquid crystal material, the liquid crystalmaterial controllable by a field.
 8. The reflector of claim 1, where thebroad bandwidth of the electromagnetic spectrum is in the visibleportion of the electromagnetic spectrum.
 9. The reflector of claim 1,where the broad bandwidth of the electromagnetic spectrum is in theultraviolet portion of the electromagnetic spectrum.
 10. The reflectorof claim 1, wherein a large plurality of the sheets are suspended in atransparent medium, the sheets being of micron size.
 11. The reflectorof claim 10, wherein the transparent medium is a fluid medium.
 12. Thereflector of claim 11, wherein the orientation of the sheets may becontrolled by field, whereby the plurality of sheets may transmit the EMradiation.
 13. The reflector of claim 12, wherein the orientation of thesheets may be controlled by field, whereby the plurality of sheets mayscatter the EM radiation.
 14. The reflector of claim 12, wherein theorientation of the sheets may be controlled by field, whereby theplurality of sheets may reflect the EM radiation in a coherent manner.15. An electro-optical glazing structure having reflection andtransmission modes of operation for selectively reflecting andtransmitting a broad band of electromagnetic radiation, respectively,the electromagnetic radiation having a first and a second linearpolarization, the electro-optical glazing structure comprising: anelectro-optical glazing panel having first and second optical states ofoperation; and optical state switching means for switching theelectro-optical panel to the first optical state of operation in orderto induce the electro-optical glazing structure into the reflection modeof operation, and for switching the electro-optical panel to the secondoptical state of operation in order to induce the electro-opticalglazing structure into the transmission mode of operation, wherein theelectro-optical panel comprises: a sheet having a large plurality ofpairs of layers parallel to a surface of the sheet, each pair of layershaving a difference between the transparent polymer materials in eachlayer of the pair, the difference being in the index of refraction forelectromagnetic radiation having the first linear polarization, whereinthere is little difference in the index of refraction forelectromagnetic radiation having the second linear polarization, thetotal thickness of each pair of layers in the large plurality of layersvarying non linearly across the sheet.
 16. The electro-optical glazingstructure of claim 15, wherein the electro-optical panel furtherreflects circularly polarized electromagnetic radiation.
 17. Theelectro-optical glazing structure of claim 16, wherein theelectro-optical panel further comprises a cholesteric liquid crystal(CLC) material.
 18. The electro-optical glazing structure of claim 15,wherein the electro-optical panel selectively transmits and reflectselectromagnetic radiation of a first bandwidth of the EM spectrum,further comprising a reflector of EM radiation which reflects radiationin a second bandwidth of the EM spectrum, the reflector of EM radiationwhich reflects radiation in a second bandwidth comprising a sheet havinga large plurality of pairs of layers parallel to a surface of the sheet,each pair of layers having a difference in the index of refractionbetween the materials in each layer of the pair.
 19. The electro-opticalglazing structure of claim 18, wherein the reflector of EM radiationwhich reflects radiation in a second bandwidth has total thickness ofeach pair of layers in the large plurality of layers varying nonlinearly across the sheet.
 20. The electro-optical glazing structure ofclaim 15, further comprising a controllable scattering layer.
 21. Theelectro-optical glazing structure of claim 20, wherein the controllablescattering layer comprises a fluid medium containing a large pluralityof anisotropically shaped objects for controllably scattering light, theorientation of anisotropically shaped objects controllable by a field.22. The electro-optical glazing structure of claim 20, wherein thecontrollable scattering layer comprises a polymer medium containing alarge plurality of inclusions, each inclusion containing liquid crystalmaterial, the liquid crystal material controllable by a field.
 23. Theelectro-optical glazing structure of claim 20, wherein the controllablescattering layer comprises a mixture of a polymer medium and a liquidcrystal material, the liquid crystal material controllable by a field.24. The electro-optical glazing structure of claim 15, wherein theelectro-optical panel selectively transmits and reflects electromagneticradiation of a first bandwidth of the EM spectrum, further comprising areflector of EM radiation which reflects radiation in a second bandwidthof the EM spectrum, the reflector of EM radiation which reflectsradiation in a second bandwidth comprising CLC layers of differenthandedness.
 25. The electro-optical glazing structure of claim 24, theCLC layers have a non linear pitch distribution.
 26. The electro-opticalglazing structure of claim 3, further comprising a controllableabsorbing layer.
 27. The electro-optical glazing structure of claim 1,wherein the means for further controlling the electromagnetic radiationcomprises an absorbing layer for controllably absorbing light.
 28. Theelectro-optical glazing structure of claim 27, wherein absorbing layeris an electrochromic absorbing layer.